The present invention relates to a gas detecting sensor for detecting the partial pressure of oxygen gas contained within exgaust gases discharged from an internal combustion engine and for measuring air/fuel ratio of the combustion mixture to be supplied into the internal combustion engine.
Recently, the so-called "lean burn system" that is, the system of operating an internal combustion engine with an air/fuel ratio larger than the stoichiometric value thereof, in order to reduce harmful components contained within the exhaust gases and lower the fuel consumption, has been proposed.
The above described "lean burn system" requires a detecting means for accurately detecting the air/fuel ratio in a range of lean mixtures.
U.S. Pat. No. 3,933,028 and U.S. Pat. No. 4,012,709 show examples of such a detecting sensor as described above.
These detecting sensors are provided with a sensing element made of cobalt monoxide (CoO) or an alloy of cobalt monoxide (CoO) and magnesium monoxide (MgO). These detecting sensors are further provided with a heating means for heating and maintaining the sensing element at a predetermined temperature, for example 900.degree. C. in order to prevent cobalt monoxide from changing into tricobalt tetroxide (Co.sub.3 O.sub.4) and to compensate for any output fluctuation caused by the temperature change.
These conventional detecting sensors have a problem in that the size of the sensing element becomes large.
In order to solve the problem of the conventional detecting sensor, the present inventors have discovered a sensor wherein on a base plate made of a material containing alumina (Al.sub.2 O.sub.3) as a main ingredient, a film shaped sensing element and film shaped electrodes are formed.
However, the present inventors have found another problem in the sensor having the above described structure.
Namely, when the device provided with the sensing element made of CoO and the base plate made of alumina is heated, the reaction of CoO+Al.sub.2 O.sub.3 .fwdarw.CoAl.sub.2 O.sub.4 occurs at about 950.degree. C., so that CoO of the sensing element is changed into CoAl.sub.2 O.sub.4.
On the other hand, since the phase of CoO is changed into Co.sub.3 O.sub.4 in an oxidizing atmosphere, the sensing element made of CoO must be maintained at a temperature above 900.degree. to 950.degree. C. in order to prevent such a phase change of the CoO.
However, at such a high temperature, CoO reacts on alumina gradually changing it into CoAl.sub.2 O.sub.4, consequently, the sensing element made of CoO loses its function.
Furthermore, as described above, the sensing element and the base plate cannot be sintered at a high temperature. Therefore, a sufficiently high adhering strength of the sensing element to the base plate cannot be obtained.
The sensing element for detecting the partial pressure of oxygen gas contained within the exhaust gases in fuel lean combustion can be also made of nickel monoxide, a perovskite type double oxide a expressed ABO.sub.3, wherein A contains lanthanum and/or B contains a cobalt other than cobalt oxide monoxide.
The present inventors have also found that the sensor provided with the sensing elements made of this material have the same problem as that provided with the sensing element made of cobalt monoxide.
Accordingly, one object of the present invention is to provide a durable gas detecting sensor of which the sensing element does not react on the material of the base plate member to cause a phase change at a high temperature.
Another object of the present invention is to provide a compact gas detecting sensor wherein a film shaped sensing element is formed on a plate shaped base plate member.
Still another object of the present invention is to provide a gas detecting sensor wherein the sensing element and the base plate member are integrally sintered through a substantially non-reactive material.